The general objective of the proposed research is to measure the extent and rate of decay of remembered dose in four normal tissues that express their injury at different times after irradiation and for the late fibrotic response in three of the tissues. More specifically, these studies will focus on the acute response in skin and intestine, the late response in lung and kidney, and the later appearing damage in bowel (stenosis), lung (fibrosis), and skin (contraction). The investigations will be carried ut in mice using well established radiobiological techniques. These studies should increase our understanding of the kinetics of depletion and repopulation of the target cells for acutely and late responding normal tissues and for the later fibrotic wave of damage common to all irradiated normal tissues, as well as provide insight into fundamental processes of tissue response to injury. The potential importance of these studies to clinical radiotherapy is that they should provide radiotherapists with guidelines for designing treatment protocols when retreatment of a previously irradiated site is necessary.